Interpretive Summary: Imidacloprid is an insecticide used to control biting and sucking insects that is very persistent in the soil with a half-life often greater than 100 days. The mechanisms of imidacloprid degradation are poorly understood and imidacloprid-degrading soil microorganisms have not been previously described. Our objectives were to discover, isolate, and characterize microorganisms capable of degrading imidacloprid in soil. We isolated a bacterial strain (PC-21) that transformed imidacloprid to several metabolites. Strain PC-21 could not use imidacloprid as source of carbon, but could utilize the insecticide as a nitrogen source. The requirement of an additional carbon source is consistent with the slow rates of degradation observed in soil. Strain PC-21 was identified as a species of Leifsonia by PCR amplification of a 500 bp sequence of 16s rRNA. The research contributes to our basic knowledge of imidacloprid behavior in the environment and will be of interest primarily to scientists.

Technical Abstract:
Imidacloprid (1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine), a chloronicotinyl insecticide used to control biting and sucking insects, is very persistent in the soil with a half-life often greater than 100 days. Although a few soil metabolites have been reported in the literature, there are no reports of imidacloprid-degrading soil microorganisms. Our objectives were to discover, isolate, and characterize microorganisms capable of degrading imidacloprid in soil. Two soil-free stable enrichment cultures in nitrogen (N)-limited media were obtained that degraded 19 mg liter (L)**-1 (43%) and 11 mg L**-1 (16%) of the applied imidacloprid, and produced about 19 mg L**-1 6-chloronicotinic acid in three weeks. Enrichment media without microorganisms had no loss of imidacloprid. Strain PC-21, obtained from the enrichment cultures, degraded 37% to 58% of 25 mg L**-1 imidacloprid in tryptic soy broth containing 1 g L**-1 succinate and D-glucose at 27 degrees carbon (C) incubation over a period of three weeks. Trace amounts of NO3-/NO2- were produced and six metabolites were characterized by high performance liquid chromatography (HPLC) using **14C-methylene-imidacloprid and LC-MS. Two of the metabolites were identified as imidacloprid-guanidine and imidacloprid-urea by HPLC standards and LC-MS. 6-chloronicotinic acid was not produced. Less than 1% of the applied **14C was incorporated into the microbial biomass and no **14CO2 (carbon dioxide) was detected. Strain PC-21, identified as a species of Leifsonia by PCR amplification of a 500 bp sequence of 16s rRNA, cometabolized imidacloprid.